The present invention relates generally to the field of implantable stimulators and more particularly to cardiac pacemakers and implantable antiarrhythia devices.
Atrial fibrillation or flutter is a common arrhythmia that is typically controlled by means of drug therapy. Over the years, there have been proposals for atrial cardioverters and defibrillators, which, in a fashion similar to the implantable cardioveter/defibrillators presently available for use in the ventricle, would terminate atrial fibrillation or flutter by means of a high voltage electrical shock.
Some retrospective studies have indicated that the presence of a pacemaker in the atrium (for example an AAI, VDD or DDD pacemaker) may reduce the incidence of atrial fibrillation, by maintaining a regular atrial rate. For example, see Lamas, G. A., et al. PACE, Vol. 15, August, 1992, pp. 1109-1113. In such applications, pacing leads are typically placed in the right atrial appendage.
It has also been proposed to reduce the incidence of ventricular fibrillation in the ventricle by using multiple site pacing. For example, in U.S. Pat. No. 3,937,226, issued to Funke, multiple electrodes are provided for location around the ventricles. In response to a sensed depolarization following a refractory period, at any of the electrodes, all electrodes are paced. All electrodes are similarly paced in the absence of sensed depolarizations for a period of 1000 ms. U.S. Pat. No. 4,088,140 issued to Rockland et al discloses a similar device, in which a pacing pulse is delivered only to a single electrode in response to a failure to sense during a 1000 ms period, and delivery of pacing pulses to multiple electrodes is triggered in response to sensed depolarizations occurring between 150 and 500 ms following delivery of a previous sensed depolarization or pacing pulse. U.S. Pat. No. 4,354,497, issued to Kahn adds sensing electrodes adjacent the septum of the heart and delivers pacing pulses to multiple electrodes spaced around the ventricles in response to sensed depolarizations at the ventricular electrodes which are not preceded by depolarizations sensed at the septum electrodes.
Cardiac potential mapping studies have shown that a portion of the atrium known as the "triangle of Koch" frequently exhibits double spiked and fractionated electrograms and may be subject to reduced conduction speed. For example, see Cossio, F. G. et al., American Journal of Cardiology, 1988, 61: 775-780, Kline, G. J. et al., American Journal of Cardiology, 1968; 57: 587-591. The triangle of Koch is a portion of the right atrium, containing the AV node, limited distally by the septal attachment of the tricuspid valve and proximally by the sinus septum. Cardiac mapping studies may also be employed to identify other areas of the atrium which display refractory periods longer than in the remainder of the atrium, in individual patients.